This invention relates to systems and methods for mixing fluids, and particularly medical fluids. More specifically, the invention relates to improvements in syringe-to-syringe mixing systems.
Several systems have been developed for on-site mixing and dispensing multi-part medical and dental compositions. One system uses dual-cartridge syringes with static mix tips. These systems are generally not adequate for mixing polymers with high mix ratios. A further drawback is that a considerable amount of material is wasted in the mix tip, which may not be problematic for low cost fluid compounds but is potentially prohibitive for expensive materials, such as an injectable disc nucleus material.
Another system, known as a continuous flow system, uses an electromechanical apparatus that drives a mix tip for controlled mixing of the fluids. Continuous flow systems are best suited for “assembly line” production and are often too expensive for mixing single batches of fluid compounds.
A system that is very compatible for mixing small batches includes two medical syringes connected by an adapter so that fluids can be pushed back and forth between the syringes. This type of system includes two syringes coupled by an adapter. The adapter includes a uniform passageway that allows flow of fluid from one syringe to the other as the plungers of the syringes are alternately depressed.
Syringe-to-syringe adapters have been used to couple a large reservoir syringe with a small dose syringe to simply transfer fluid from one to the other. These adapters have also been used to sequentially couple different syringes to a single syringe, with each of the syringes carrying a different fluid or a granular compound to mix with the fluid in the single syringe. In some cases, the two syringes contain different fluids that must be thoroughly mixed. This mixing occurs by alternately depressing the plungers of the opposing syringes so that the fluids flow back and forth through the adapter. Once the fluid transfer or mixing is complete, the syringes are uncoupled and one or both of the syringes can be used as an applicator or injection device.
For many types of fluids and fluid compounds, this mixing approach is sufficient. For instance, many emulsions are prepared through syringe-to-syringe mixing. In these prior devices, the constant diameter passageway in the adapter allows full uniform flow of the fluid through the adapter, and the resultant mixture is complete enough for the particular medical application. One drawback of these prior systems is that they require relatively high plunger forces when mixing viscous fluids, which can lead to user fatigue. Another problem is that it is time consuming to achieve uniform distribution of micro-droplets within a fluid mixture.
Furthermore, in certain medical applications, the degree of mixing that can be accomplished is less than optimum, particularly where high mix ratios are involved. For instance, certain injectable disc nucleus (IDN) compositions can have mix ratios between two constituents (i.e., polymer and cross-linker) greater than 10:1, and even greater than 100:1. The entire composition fails if the lower concentration constituent (such as the cross-linker in the case of an IDN) is not fully mixed within the other constituent (the polymer).
This mixing problem is also critical where the fluids combine to form a curable composition. In this case, as the different fluids are mixed they begin to cure, congeal or harden. For some materials, the curing time is sufficiently long so that the mixture can be cycled back and forth between the syringes enough times to ensure complete mixing of the constituents. For instance, many bone cements can be mixed using these types of prior devices.
However, the time necessary to achieve complete mixing is prohibitive for some curable materials that cure relatively quickly. If these types of materials are not dispensed in a timely manner, the mixture is worthless. For example, one type of chemical composition known as a hydrogel is formed by mixing a polymer with a cross-linker. The resulting mixture starts to cure immediately when the constituents come into contact. For some hydrogels, the curing time is under two minutes. In these cases, it is imperative that the fluid mixing occur as quickly and completely as possible so that the surgeon has enough time remaining to inject the hydrogel at the surgical site.
The short curing times essentially prohibit mixing the constituents in any system other than a system that permits immediate injection of the mixture. In other words, syringe-to-syringe mixing is the most viable alternative for fluid compounds having short curing times.
Consequently, there is a need for a syringe-to-syringe system that yields complete mixing in conditions that include one or more of the following parameters:
High mix ratios (e.g., much greater than 10:1);
Immiscible fluids;
Rapidly curing polymers; and
High viscosity fluids.